Numerous technologies which will utilize coal are being developed as a way of improving the efficiency of fossil fuel systems. Typical of these technologies are combined-cycle coal gasification, combined-cycle pressurized fluidized bed combustion, direct coal-fired gas turbines, and coal gasification molten carbonate fuel cell systems. An important aspect of all of these technologies is the matter of handling the coal and combustion products in an environmentally acceptable manner. In many of these systems, the combustion (or other product) gases are to be directed to turbines for the production of electrical energy. However, sulfur, alkali metals, NO.sub.x and solid particulates must be removed from the gas stream to protect metallic components of the turbine system from corrosion and erosion. Some of these constituents can be removed chemically, and others must be physically removed.
Filter units are conventionally utilized for the physical removal of particulates in gaseous systems. However, those that are known in the past for many applications will not be satisfactory for these new techniques of fossil energy production. For example, most of the prior art filters cannot withstand the elevated temperatures approaching 1000 degrees C. Others are conventionally manufactured from materials that will not withstand the corrosive and erosive effects of the constituents of the gases. In order to utilize these conventional filter materials, the gases must be cooled prior to the filtering step. Then the gases are reheated for use in a turbine thereby significantly reducing the conversion efficiency of the system.
A number of techniques have been investigated for the removal of the particulates from high temperature gases (up to about 600 degrees C). For example, ceramic bag filters fabricated from ceramic cloth or felts have been utilized. These are usually fitted over some form of frame, e.g., a wire mesh, to provide stability during use. A typical method of cleaning these filters, as in any other type of filtering system, is to pass a gas in a reverse direction for a short period of time (pulse blow-back). However, the ceramic filters of the prior art are often destroyed by this cleaning as the fibers within the felt or cloth tend to move, or even break; thus, a hole is created. While this type of filter can be used for a period of time, the filter efficiency becomes greatly reduced as the holes are created.
A second type of filter, referred to as a dense "candle" filter, consists of silicon carbide particulates bonded together by a clay matrix which becomes a glass upon firing. This structure results in thick walls making the filters relatively heavy. Furthermore, because of the high density and thickness, these filters are subject to thermal shock, particularly during the pulse cleaning step.
Typically, these prior art ceramic filters have been fabricated from SiC or aluminosilicate fibers of a form that is relatively impure, often containing additives to aid in forming the filters. This creates a potential problem of chemical reactions with constituents of the gas, such as with sodium. For example, sodium is known to form a relatively low melting eutectic with silicon oxides at temperatures in the range of potential operation of the filter; thus, creating an additional problem with these filters of the prior art.
Although the following art does not deal with the preparation of ceramic fiber reinforced filters, the use of chemical vapor deposition techniques are taught with regard to fiber-containing preforms. U.S. Pat. No. 4,580,524, issued to W. J. Lackey, et al. on Apr. 8, 1986, describes a process of preparing fiber-reinforced composites (all interstices filled) by chemical vapor deposition. Numerous references cited therein relate to similar technologies for producing fully infiltrated composites and equipment to effect the same. Some of the references deal with the infiltration of yarn, woven cloth (carbon) and three dimensional preforms to fully fill the pores thereof. Various carbides, including SiC are used.
Other references that are generally related to the present invention are U.S. Pat. Nos. 4,275,095 and 4,397,901, issued to J. W. Warren on June 23, 1981 and Aug. 9, 1983, respectively. The technology of these patents is also concerned with infiltration, by CVD techniques, of a fibrous substrate. These teach the use of a pyrolytic carbon sheath formed on each fiber before the deposit of a metallic refractory.
Accordingly, it is an object of the present invention to provide a filter for the removal of particulates from high temperature fluids, such as gases, that can be utilized at the temperature needed for operation of a power-producing turbine.
It is another object of the invention to provide a ceramic fiber filter that will withstand periodic cleaning as with pulse blow-back.
Another object of the present invention is to provide a filter material that can be formed into shapes most practical for the removal of particulates, with the resultant filter unit having sufficient strength such that metallic or other structural components are not needed for support.
A further object of the invention is to provide a filter that is substantially resistant to corrosion, erosion and chemical attack from the constituents of the gas during high temperature operation for the removal of particulates from the gas stream.
These and other objects of the present invention will become apparent upon a consideration of the drawings referred to hereinafter and the complete description thereof.